dfp: Further decimal_real_to_integer fixes [PR120631]

dfp: Further decimal_real_to_integer fixes [PR120631]

Unfortunately, the following further testcase shows that there aren't
problems only with very large precisions and large exponents, but pretty
much anything larger than 64-bits.  After all, before _BitInt support dfp
didn't even have {,unsigned }__int128 <-> _Decimal{32,64,128,64x} support,
and the testcase again shows some of the conversions yielding zeros.
While the pr120631.c test worked even without the earlier patch.

So, this patch assumes 64-bit precision at most is ok and for anything
larger it just uses exponent 0 and multiplies afterwards.

2025-06-19  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/120631
	* dfp.cc (decimal_real_to_integer): Use result multiplication not just
	when precision > 128 and dn.exponent > 19, but when precision > 64
	and dn.exponent > 0.

	* gcc.dg/dfp/bitint-10.c: New test.
	* gcc.dg/dfp/pr120631.c: New test.

(cherry picked from commit e2eb9da5546b5e2fccb86586cda3beee8f69f5c9)